1. Field of the Invention
The present invention relates to a webbing take-up apparatus for taking up a webbing which is provided in a vehicle or the like to restrain a vehicle occupant.
2. Description of the Related Art
In a webbing take-up apparatus used in an automobile or the like, a lock mechanism is provided to prevent a webbing from being pulled out when the automobile or the like decelerates abruptly, thereby securely restraining (or holding) a vehicle occupant.
In such a lock mechanism, when an abrupt deceleration is detected by an acceleration sensor, locking means is activated in response thereto to be engaged with inwardly extending ratchet teeth formed in one of the side walls of a frame, thereby preventing a webbing take-up shaft from rotating in a webbing pull-out direction.
In the above-described webbing take-up apparatus, when the webbing is pulled out by the body of a vehicle occupant due to the inertia during an abrupt deceleration, the pulling force acting on the webbing is transmitted to the frame in which the ratchet teeth are formed. Accordingly, it is required to increase the strength of the frame of the webbing take-up apparatus by increasing the thickness of the frame or making the frame of a special material.
To overcome this problem, there have been proposed improved webbing take-up apparatuses, such as one disclosed in Japanese Patent Publication No. 59-21624, in which inwardly extending ratchet teeth are formed in both side walls of the frame, and a pair of lock means for these ratchet teeth in the both side walls are provided to prevent a webbing take-up shaft from rotating in the webbing pullout direction.
In the webbing take-up apparatus disclosed in Japanese Patent Publication No. 59-21624, inwardly extending ratchet teeth are formed in both side walls of the frame. Also, the take-up shaft is rotatably supported by pivot bearings provided in both sides of an outer cover for the frame, via bearings formed in the both side walls and extensions provided at both axial ends of the take-up shaft.
Cylindrical pins are provided on the axial end portions of the take-up shaft such that they are situated near a center axis and project from the axial end portions. Lock plates having outer teeth are rotatably supported by the cylindrical pins. A slide surface is formed on the end portion of each lock plate, and one end of a control lever contacts the slide surface to control the movement of the lock plate. When the control lever is operated, the lock plate is rotated while sliding relative to the control lever.
The control levers provided on both sides of the take-up shaft are connected to each other via a connecting rod provided inside the take-up shaft. The other end of one control lever, i.e., the end which does not contact the lock plate is provided with another slide surface.
On the extension provided at one axial end of the take-up shaft, an inertia plate having teeth at the outer circumferential surface thereof is supported for relative rotation with resect to the take-up shaft. From the inner end surface of the inertia plate, a connecting projection projects toward the interior of the frame for contact with the slide surface of the control lever. Also, an acceleration sensor is disposed below the inertial plate. When the acceleration sensor detects an abrupt deceleration of the vehicle, it stops the inertial plate.
During an abrupt deceleration of the vehicle, the webbing is pulled out by the inertia of the body of a vehicle occupant so that the take-up shaft is forced to rotate in the pull-out direction of the webbing. At this time, since the connecting projection provided on the inertia plate is stopped due to the stoppage of the inertia plate, a slide surface formed at the other end of the control lever contacts the connecting projection.
As a result, the control lever, while resisting the connecting projection, rotates in a predetermined direction about the connecting rod inside the take-up shaft and rotates another control lever via the connecting rod inside the take-up shaft.
Both the lock plates contacting the control levers through the respective slide surfaces move in a predetermined direction, so that they respectively mesh with inner ratchet teeth formed in both side walls of the frame, thereby stopping the rotation of the take-up shaft.
As described above, in the conventional webbing take-up apparatus having the above-described structure, the take-up shaft is stopped by the ratchet teeth formed in both side walls and the lock plates each engagable with the ratchet teeth. Accordingly, when the webbing is pulled out by the body of a vehicle occupant due to the inertia during an abrupt deceleration of the vehicle, the pulling force acting on the webbing is transmitted to both side walls of the frame. This eliminates the necessity of increasing the thickness of the frame, and reduces the weight of the webbing take-up apparatus. Also, since material having a very high strength is not needed, the webbing take-up apparatus can be manufactured at reduced costs.
However, in the above-described webbing take-up apparatus, the connecting projections and the control levers establish sliding contact therebetween to transmit movement from the former to the latter, and the control levers and the lock plates establish sliding contact therebetween to transmit movement from the former to the latter. Accordingly, there is a possibility that the lock timing of each lock plate becomes uncontrollable due to the dimensional errors and finish of those parts, and changes in the quality of the parts due to elapse of time, especially changes in the quality of the slide surfaces caused by wear.
In addition, since the lock mechanism for locking the take-up shaft which takes up the webbing is composed of many parts most of which are not directly connected to each other, each part involves an error in its movement. The error of movement of each part is transmitted to a downstream part so that the error is amplified. This increases the possibility that the lock timing of each lock plate becomes uncontrollable.
When the lock timing of each lock plate becomes uncontrollable, there occurs a problem that outer teeth of the pair of lock plates do not correctly mesh with the ratchet teeth formed in the side walls.
If the machining accuracy of each part is increased, the lock timing of each lock plate of the above-described webbing take-up apparatus can be securely controlled. However, this increases manufacturing costs.